1. Field of the Invention
This invention relates to an electron source and an image-forming apparatus realized by using the same and, more particularly, it relates to an electron source comprising a plurality of surface conduction electron emitting devices and an image-forming apparatus realized by using the same.
2. Related Background Art
Thermoelectron sources and cold cathode electron sources are known as two types of electron emitting devices. Electron emitting devices that can be used for cold cathode electron sources include those of field emission type (hereinafter referred to as FE type), metal/insulation layer/metal type (hereinafter referred to as MIN type) and surface conduction type.
Examples of FE type devices are proposed in W. P. Dyke & W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956), A. Spindt, "PHYSICAL Properties of thin-film field emission cathode with molybdenum cones", J. Appln. Phys., 47, 5248 (1976). An MIN type device is disclosed in C. A. Mead, "The tunnel emission amplifier", J. Appln. Phys., 32, 646 (1961). A surface conduction electron-emitting device is proposed in M. I. Elinson, Radio Eng. Electron Phys., 10 (1965).
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of a SnO.sub.2 thin film for a device of this type, the use of an Au thin film is proposed in G. Dittmer: "Thin Solid Films", 9, 317 (1972), whereas the use of an In.sub.2 O.sub.3 /snO.sub.2 Thin film and that of a carbon thin film are discussed respectively in M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975) and H. Araki et al.: "Vacuum", Vol. 26, No. 1, p. 22 (1983).
FIG. 31 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell. In FIG. 31, reference numerals 311, 313 and 314 respectively denote an insulator substrate, an electron-emitting region and a thin metal oxide film including said electron-emitting region, whereas reference numerals 315 and 316 denote device electrodes that are made of a material common with that of the thin film 314. Referring to FIG. 31, the thin metal oxide film has a length L.sub.1 of 0.5 to 1 mm and a width W of 0.1 mm. Note that the electron-emitting region 313 is only very schematically shown there.
A surface conduction electron-emitting device having a configuration as described above is normally prepared by producing an H-shaped thin metal oxide film, part of which eventually makes an electron-emitting region, on an insulator substrate 311 by means of sputtering and then the thin oxide film is partly transformed into an electron-emitting region 313 by using a process of preliminarily energizing the thin film which is generally referred to as "forming". In a forming process, a voltage is applied to given opposite ends of a thin film for preparing an electron-emitting region so that a part of the thin film may be destructed, deformed or transformed to become an electron-emitting region 313 which is electrically highly resistive as a result of energizing.
The electron-emitting region 313 of the surface conduction electron-emitting device produced by the forming process normally has fissures in part of the thin film and electrons are emitted from those fissures when a voltage is applied to the thin film 314 to cause an electric current flow therethrough.
However, known surface conduction electron-emitting devices having a configuration as described above have a number of problems to be solved if they are to be used for practical applications.
Surface conduction electron-emitting devices are, on the other hand, advantageous in that they can be formed in arrays in great numbers over a large area because they are structurally simple and hence can be manufactured at low cost in a simple way. In fact, many studies have been made to exploit this advantage and applications that have been proposed as a result of such studies include charged particle beam sources and electronic displays. A large number of surface conduction electron-emitting devices can be arranged in an array to form a matrix pattern that operates as an electron source, where the devices of each row are wired in parallel and the rows are regularly arranged to form the array. (See, for example, Japanese Patent Application Laid-Open No. 64-31332 in the name of the same applicant as the present case.)
As for image-forming apparatuses comprising surface conduction electron-emitting devices such as electronic displays, although flat panel displays using a liquid crystal have gained popularity in place of CRT in recent years, such displays are not without problems. One of the problems is that a light source is needed because those displays are not of emission type. An emission type display can be realized using an electron source formed by arranging a large number of surface conduction electron-emitting devices in combination with a fluorescent body that is induced to selectively shed visible light by electrons emitted from the electron source. With such an arrangement, an emission type display apparatus having a large display screen and enhanced display capabilities can be manufactured relatively easily at low cost. See, for example, the U.S. Pat. No. 5,066,883 by the same applicant as the present case.
Incidentally, Japanese Patent Application Laid-Open Nos. 1-283749, 1-257552 and 64-31332 disclose different but similar electron sources that can be used for an image-forming apparatus comprising a plurality of electron-emitting devices. In those electron sources, the plurality of electron-emitting devices are arranged to form a matrix, where the electron-emitting devices of each row are connected in parallel by common wires while control electrodes (grids) are disposed perpendicular to the common wires in a space between the electron source and the fluorescent body so that any of the devices may be selected by applying selectively an appropriate drive signal to the common wires as rows and the control electrodes as columns of the matrix. FIG. 32 of the accompanying drawings schematically shows a plan view of part of an electron source of the type under consideration comprising a plurality of surface conduction electron-emitting devices. Referring to FIG. 32, a plurality of electron-emitting devices 320 are arranged on a substrate and the devices of each row are connected in parallel by a pair of common wires, e.g. common wires 321 and 322, and a grid GR having a number of electron passing holes Gh is arranged for each column of devices perpendicularly to the common wires 321, 322 and above the electron-emitting devices 320 on the substrate.
However, an image-forming apparatus comprising an electron source composed of a plurality of surface conduction electron-emitting devices and a fluorescent body disposed as opposing the electron source is not without problems. Though the surface conduction electron-emitting devices in such an apparatus can be selected and the selected devices can be controlled for electron emission with an image-forming apparatus of the above identified type, this apparatus is not simple. In other words, grids are indispensably needed and arranged along the columns of devices to select a particular device and cause the fluorescent body to emit light selectively at a controlled brightness.
An image-forming apparatus as described above is therefore accompanied by difficulties that commonly appear in the course of manufacture including the difficulty of aligning surface conduction electron-emitting devices and grids and accurately controlling the distance separating the grids and the surface conduction electron-emitting devices. In an attempt to bypass these difficulties, the inventors of the present patent application have already proposed a novel structure wherein grids are laminated on the surface conduction electron-emitting devices. (See Japanese Patent Application Laid-Open No. 3-20941.)
In such a structure, however, the process of manufacturing a plurality of known surface conduction electron-emitting devices involves a step of forming device electrodes and electron-emitting regions in addition to the ordinary steps of wiring as well as the step of preparing grids and therefore, the entire process is cumbersome and complicated.